add explanation and examples of file system objects

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+# File System Object
+
+The Nix store uses a simple file system model for the data it holds in [store objects](store.md#store-object).
+
+Every file system object is one of the following:
+
+ - File: an executable flag, and arbitrary data for contents
+ - Directory: mapping of names to child file system objects
+ - [Symbolic link][symlink]: may point anywhere.
+
+We call a store object's outermost file system object the *root*.
+
+    data FileSystemObject
+      = File      { isExecutable :: Bool, contents :: Bytes }
+      | Directory { entries :: Map FileName FileSystemObject }
+      | SymLink   { target :: Path }
+
+Examples:
+
+- a directory with contents
+
+    /nix/store/<hash>-hello-2.10
+    ├── bin
+    │   └── hello
+    └── share
+        ├── info
+        │   └── hello.info
+        └── man
+            └── man1
+                └── hello.1.gz
+
+- a directory with relative symlink and other contents
+
+    /nix/store/<hash>-go-1.16.9
+    ├── bin -> share/go/bin
+    ├── nix-support/
+    └── share/
+
+- a directory with absolute symlink
+
+    /nix/store/d3k...-nodejs
+    └── nix_node -> /nix/store/f20...-nodejs-10.24.
+
+A bare file or symlink can be a root file system object.
+Examles:
+
+    /nix/store/<hash>-hello-2.10.tar.gz
+
+    /nix/store/4j5...-pkg-config-wrapper-0.29.2-doc -> /nix/store/i99...-pkg-config-0.29.2-doc
+
+Symlinks pointing outside of their own root or to a store object without a matching reference are allowed, but might not function as intended.
+Examples:
+
+- an arbitrarily symlinked file may change or not exist at all
+
+    /nix/store/<hash>-foo
+    └── foo -> /home/foo
+
+- if a symlink to a store path was not automatically created by Nix, it may be invalid or get invalidated when the store object is deleted
+
+    /nix/store/<hash>-bar
+    └── bar -> /nix/store/abc...-foo
+
+[symlink]: https://en.m.wikipedia.org/wiki/Symbolic_link